The present invention relates to optical apparatus for measuring the turbidity of fluids; and particularly to circuitry for processing signals from light detectors utilized in such apparatus to produce a measurement of the turbidity.
Turbidity is an optical characteristic of a fluid that is related to the presence, nature and amount of suspended matter or particles which scatter light in an otherwise pure fluid. Turbidity may be sensed by instruments commonly known as turbidimeters which measure the characteristic in terms of the amounts of light which are transmitted directly through and scattered by the fluid.
One type of previous turbidimeter utilized a pair of DC light sources to produce light beams of constant intensity and two detectors in which each detector was aligned with a different light source. The light sources were alternately energized and the amounts of light detected by the aligned and unaligned detectors were compared. The DC detector signals produced during energization of each light source were processed to derive a turbidity value, as defined by the U.S. Environmental Protection Agency. Such photoelectric instruments permit turbidity measurements to be conducted on static fluids or those which flow continuously between the emitters and detectors.
The previous turbidimeters used DC amplifiers in the measurement circuitry. Significant errors often existed due to variation of the amplifier's D.C. offset and gain resulting from age and varying operating conditions. As a consequence conventional turbidimeters require frequent recalibration to known turbidity standards. Turbidimeters which used multiple sources and detectors to perform the measurement by comparing the signals from the aligned and unaligned detectors, eliminated much of the gain error. However these devices did not correct for offset errors.